Some handheld computing devices such as smart phones can provide a variety of different optical functions such as one-dimensional (1D) or three-dimensional (3D) gesture detection, 3D imaging, proximity detection, ambient light sensing, and/or front-facing two-dimensional (2D) camera imaging.
TOF-based systems, for example, can provide depth and/or distance information. In general, TOF systems are based on the phase-measurement technique of emitted intensity-modulated light, which is reflected by a scene. The reflected light is imaged onto a sensor, and the photo-generated electrons are demodulated in the sensor. Based on the phase information, the distance to a point in the scene for each pixel is determined by processing circuitry associated with the sensor.
Additionally, TOF-based systems can provide depth and/or distance information via a pulse-measurement technique. The pulse-measurement technique employs an emitter and sensor as above; however, distance is determined by tallying the time for emitted light to reflect back onto the sensor.
Integrating TOF sensors into devices such as smart phones, tablets or other handheld devices, however, can be challenging for several reasons. First, space in the host device typically is at premium. Thus, there is a need to achieve accurate TOF sensors having a relatively small height. Second, the size of the dies impacts production costs. Accordingly, it is desirable to achieve TOF sensors having a relatively small foot print.
While the foregoing issues also may be applicable to other types of optical imaging or detection sensors, another potential problem is more specific to TOF sensors. In particular, the distance measurements obtained by the pixels should be robust against phase delays caused, for example, by thermal drifting effects. To address such concerns, in some TOF chips, a self-calibration of the TOF distance measurement is achieved by providing reference pixels that measure light from the illumination source. The use of such reference pixels necessitates directing some of the light from the illumination source to the reference pixels, which may need to be separated optically from the active pixels used to measure the distance to the scene.
TOF-based distance measurements via the pulsed-measurement technique should be robust against thermal drifting effects. For example, in some instances the precise time for commencement of the initial emission of light from the emitter may be obscured by thermal drifting effects.